Funnily enough this conversation did come up at a dinner party with friends last week - one friend works in a residential home in England.
She said she would not put her parents or in-laws in a home, after working in them, as long as she could physically and financially avoid it. One of her opinions was that old folk just 'give up' on life, feel like they've stepped out of society, as soon as they move in.
I used to live in a very poor African country, and although the people there had little materially, they looked after their family. To my many friends there, the idea of putting your parents in an old folks' home was abhorrent and equal to suggesting your elderly parents should live out their days as a street beggar. (Many heated discussions about selfish Westerners biting the parental hand that nurtured them, etc etc)
I agree with the poster who wrote 'you reap what you sow', and would definitely look after my parents, and at more of a push, DH's problem-strewn parents too. Happily, they're all in their 50s and more concerned with looking after their own parents for now.
I also agree that money and your house pay a massive part - a granny/grandad flat with own kitchen, entrance etc would be ideal. And their would be conditions too...
My parents would look after me in a heartbeat - only recently dd and I had to live with them for 6 months, and they were wonderful. If, God forbid, I was in a terrible accident, widowed or whatever, they wouldn't think twice about taking me in. I think it should work both ways.